Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
The use of solar cells is rapidly expanding because they provide a sustainable energy resource. Solar cells can typically be categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells.
Monocrystalline silicon (c-Si), poly crystalline (poly-Si), multicrystalline silicon (mc-Si) and ribbon silicon are the materials used most commonly in forming the more traditional wafer-based solar cells. Solar cell modules derived from wafer-based solar cells often comprise a series of about 180 and about 240 μm thick self-supporting wafers (or cells) that are soldered together. Such a panel of solar cells, along with a layer of conductive paste and/or conducting wires and bus bars deposited on its surface, is then encapsulated by polymeric encapsulants to form a solar cell assembly, which may be further sandwiched between two protective outer layers to form a weather resistant module. The protective outer layers may be formed of glass, metal sheets or films, or plastic sheets or films. In general, however, the outer layer that faces to the sunlight needs to be sufficiently transparent to allow photons to reach the solar cells.
As for the increasingly important alternative, thin film solar cells, the commonly used materials include amorphous silicon (a-Si), microcrystalline silicon (μc-Si), cadmium telluride (CdTe), copper indium selenide (CuInSe2 or “CIS”), copper indium/gallium diselenide (CuInxGa(1-x)Se2 or “CIGS”), light absorbing dyes, organic semiconductors, etc. By way of example, thin film solar cells are described in U.S. Pat. Nos. 5,507,881; 5,512,107; 5,948,176; 5,994,163; 6,040,521; 6,123,824; 6,137,048; 6,288,325; 6,258,620; 6,613,603; and 6,784,301; and U.S. Patent Application Publication Nos. 20070298590; 20070281090; 20070240759; 20070232057; 20070238285; 20070227578; 20070209699; 20070079866; 20080223436; and 20080271675. Thin film solar cells with a typical thickness of less than 2 μm are generally produced by depositing the semiconductor materials onto a substrate in multi-layers. The substrate may be formed of glass or a flexible film, and it may be referred to as a “superstate” in those modules in which it faces the sunlight. Similarly to wafer-based solar cell modules, the thin film solar cells are further encapsulated by polymeric encapsulants and sandwiched between protective outer layers. In certain modules, the only the side of the thin film solar cell that is opposite from the substrate is encapsulated by the polymeric encapsulants and further laminated to a protective outer layer. Further, conducting wirings and bus bars, metal conductive coatings, and/or metal reflector films may be deposited over the surface of the thin film solar cells and encapsulated, along with the thin film solar cells, by the encapsulants.
Within the solar cell modules, some components, such as the conducting wires and bus bars, the conductive paste that is used in wafer-based solar cell modules, the conductive coatings that are used in thin film solar cells, and the back reflector films that are used in thin film solar cell modules, may comprise metals, such as silver. Moreover, these metal-comprising component(s) may come in contact with the polymeric encapsulants. In those modules in which poly(vinyl butyral) (PVB) is used as the encapsulant material, it is found that the PVB tends to discolor over time, when in contact with an oxidizable metal component. Thus, there is a need to develop a PVB composition useful as an encapsulant material for solar cell modules that resists discoloration when in contact with oxidizable metal components over the life of the solar cell module.